The toothing(45,46) provided for each guide rail(37,38) is constructed directly on it, and the guide rail is formed so that the toothing points towards the guide plane of the gearwheel support shaft(50) defined by a slot(39,40) in the guide rail. A twist resistant profile is constructed on the ends of the support shaft and the gearwheels(51,52) are fitted on the shaft. Stop nuts(53,54) are connected to the shaft for axial locking of the gearwheels. Both guide rails are constructed directly on support beam.
